Hydrocarbon mixtures, such as crude oils and heavy fuel oils, with a general phase may be subject to physical properties changes such as solubility due to a series of operational parameters, such as temperature, pressure, and blending with different fluids such as hydrocarbon mixtures, water, and other liquids that may adversely affect the solubility of the resulting mixture, etc. Hydrocarbon mixtures may include hydrocarbons that may form hydrates when exposed to a variety of conditions, particularly a combination of lower temperature and higher pressure, in the presence of water. Hydrate solids (or crystals) may cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and risk of explosion or unintended release of hydrocarbons into the environment either on-land or off-shore.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions.
Solubility variations in hydrocarbon mixtures may have objectionable effects on the mixture as a whole, such as when impurities drop out of the general phase to form undesirable precipitates, such as flocculation of asphaltenes (forming the additional phase), such as fouling scale deposits, etc. These impurities may precipitate out of the mixture or remain suspended. While remaining as an additional phase, the impurities may aggregate into substantial masses that may foul piping, storage facilities, and processing units as well as degrade the quality of the mixture. When a hydrocarbon mixture has formed an additional phase with objectionable properties, the mixture may be characterized as “unstable” or as “demonstrating instability.”
Additives may be introduced to hydrocarbon mixtures to prevent or inhibit formation or aggregation of the additional phase (such as flocculated asphaltenes) and to restore stability to the hydrocarbon mixture. However, detection of formation of an additional phase generally must occur quickly to avoid aggregation of the additional phase into a substantial mass. On the other hand, since the additive is likely to be relatively expensive, the decision to introduce an additive, and a minimum appropriate amount of the additive, should be made judiciously. Hence, it is desirable to continuously monitor hydrocarbon mixtures for the aggregation of asphaltenes, and other substances that may form substantial masses within the hydrocarbon mixture, so that additives may be introduced quickly to mitigate problems due the flocculation of substances and their aggregation. It is also desirable to control or prevent the formation of an additional phase by identifying ratios of blend components such that stability of the hydrocarbon mixture is preserved.